Numbers 5:18 - Matthew Poole's English Annotations on the Holy Bible

Before the Lord; before the tabernacle, with her face towards the ark. Uncover the woman's head; partly, that she might be made sensible how manifest she and all her ways were to God, and that she might be more visible to the congregation, that her shame might be greater if she were guilty; partly, in token of her sorrow either for her sin, or at least for any cause of suspicion which she had given; partly, as a sign that she was after a sort deprived of the help and protection of her husband, which the covering of the woman's head signified, 1 Corinthians 11:5-7,10, and that she was neither virgin nor loyal with, for the heads of both these used to be covered. In her hands, that she herself might offer it, and therefore call God to be witness of her innocency. Bitter; so called either from the bitter taste which the dust gave it, or from the bitter effects of it upon her if she were guilty. Compare Exodus 32:20. That causeth the curse; not by any natural power, but by a supernatural efficacy ordained and wrought by God for her punishment, and for the terror and caution of others.